Chromosome organization. Arrangement of DNA in 100A nucleohistone fiber: Up to now preparations for electron microscopy had to be dried, causing possible artifacts. With an environmental chamber in the Million volt EM wet, undehydrated specimens can be imaged with good resolution. We shall use this on 100A and 200A chromatin fibers from various sources. Patterns of nucleohistone fibers in chromomeres and mitotic chromosomes: Chromomere structure will be studied with the Million Volt EM in polytene chromosomes of the macronucleus (Euplotes) and in various tissues of Drosophila larvae. Structure of mitotic chromosomes will be analyzed for several species with small chromosomes such as Yeast, Drosophila, Daphnia, etc. Structure of active chromatin in situ will be studied in lampbrush chromosomes, puffs of polytene chromosomes and somatic nuclei in 1 sections with the High Voltage EM. Primitive Mitosis. Our work on nuclear division in Dinoflagellates, Radiolaria, Yeast, hypermastigote flagellates, sporozoa will be continued using reconstruction from serial sections and High Voltage EM of thick sections. The undersigned agrees to accept responsibility for the scientific and technical conduct of the project and for provision of required progress reports if a grant is awarded as the result of this application.